Machine vision systems use image acquisition devices that include camera sensors to deliver information on a viewed subject. The system then interprets this information according to a variety of algorithms to perform a programmed decision-making and/or identification function. For an image to be most-effectively acquired by a sensor in the visible, and near-visible light range, the subject should be properly illuminated.
In the example of symbology reading (also commonly termed “barcode” scanning) using an image sensor, proper illumination is highly desirable. Symbology reading entails the aiming of an image acquisition sensor (CMOS camera, CCD, etc.) at a location on an object that contains a symbol (a “barcode”), and acquiring an image of that symbol. The symbol contains a set of predetermined patterns that represent an ordered group of characters or shapes from which an attached data processor (for example, a microcomputer) can derive useful information about the object (e.g. its serial number, type, model, price, etc.). Symbols/barcodes are available in a variety of shapes and sizes. Two of the most commonly employed symbol types used in marking and identifying objects are the so-called one-dimensional barcode, consisting of a line of vertical stripes of varying width and spacing, and the so-called two-dimensional barcode consisting of a two-dimensional array of dots or rectangles.
By way of background FIG. 1 shows an exemplary scanning system 100 adapted for handheld operation. An exemplary handheld scanning appliance or handpiece 102 is provided. It includes a grip section 104 and a body section 106. An image formation system 151, shown in phantom, can be controlled and can direct image data to an on-board embedded processor 109. This processor can include a scanning software application 113 by which lighting is controlled, images are acquired and image data is interpreted into usable information (for example, alphanumeric strings derived from the symbols (such as the depicted two-dimensional barcode image 195). The decoded information can be directed via a cable 111 to a PC or other data storage device 112 having (for example) a display 114, keyboard 116 and mouse 118, where it can be stored and further manipulated using an appropriate application 121. Alternatively, the cable 111 can be directly connected to an interface in the scanning appliance and an appropriate interface in the computer 112. In this case the computer-based application 121 performs various image interpretation/decoding and lighting control functions as needed. The precise arrangement of the handheld scanning appliance with respect to an embedded processor, computer or other processor is highly variable. For example, a wireless interconnect can be provided in which no cable 111 is present. Likewise, the depicted microcomputer can be substituted with another processing device, including an onboard processor or a miniaturized processing unit such as a personal digital assistant or other small-scale computing device.
The scanning application 113 can be adapted to respond to inputs from the scanning appliance 102. For example, when the operator toggles a trigger 122 on the hand held scanning appliance 102, an internal camera image sensor (that is part of the image formation system 151) acquires an image of a region of interest 131 on an object 105. The exemplary region of interest includes a two-dimensional symbol 195 that can be used to identify the object 105. Identification and other processing functions are carried out by the scanning application 113, based upon image data transmitted from the hand held scanning appliance 102 to the processor 109. A visual indicator 141 can be illuminated by signals from the processor 109 to indicate a successful read and decode of the symbol 195.
In reading symbology or other subjects of interest, the type of illumination employed is of concern. Where symbology and/or other viewed subjects are printed on a flat surface with contrasting ink or paint, a diffuse, high-angle “bright field” illumination may best highlight these features for the sensor. By high-angle it is meant, generally, light that strikes the subject nearly perpendicularly (normal) or at an angle that is typically no more than about 45 degrees from perpendicular (normal) to the surface of the item being scanned. Such illumination is subject to substantial reflection back toward the sensor. By way of example, barcodes and other subjects requiring mainly bright field illumination may be present on a printed label adhered to an item or container, or on a printed field in a relatively smooth area of item or container.
Conversely, where a symbology or other subject is formed on a more-irregular surface, or is created by etching or peening a pattern directly on the surface, the use of highly reflective bright field illumination may be inappropriate. A peened/etched surface has two-dimensional properties that tend to scatter bright field illumination, thereby obscuring the acquired image. Where a viewed subject has such decidedly two-dimensional surface texture, it may be best illuminated with dark field illumination. This is an illumination with a characteristic low angle (approximately 45 degrees or less, for example) with respect to the surface of the subject (i.e. an angle of more than approximately 45 degrees with respect to normal). Using such low-angle, dark field illumination, two-dimensional surface texture is contrasted more effectively (with indents appearing as bright spots and the surroundings as shadow) for better image acquisition.
In other instances of applied symbology a diffuse direct illumination may be preferred. Such illumination is typically produced using a direct-projected illumination source (e.g. light emitting diodes (LEDs)) that passes through a diffuser to generate the desired illumination effect.
To take full advantage of the versatility of a camera image sensor, it is desirable to provide bright field, dark field and diffuse illumination. However, dark field illumination must be presented close to a subject to attain the low incidence angle thereto. Conversely, bright field illumination is better produced at a relative distance to ensure full area illumination.
Commonly assigned U.S. patent application Ser. No. 11/014,478, entitled HAND HELD SYMBOLOGY READER ILLUMINATION DIFFUSER and U.S. patent application Ser. No. 11/019,763, entitled LOW PROFILE ILLUMINATION FOR DIRECT PART MARK READERS, both by Laurens W. Nunnink, the teachings of which are expressly incorporated herein by reference, provide techniques for improving the transmission of bright field (high angle) and dark field (low angle) illumination. These techniques include the provision of particular geometric arrangements of direct, bright field LEDs and conical and/or flat diffusers that are placed between bright field illuminators and the subject to better spread the bright field light. The above-incorporated HAND HELD SYMBOLOGY READER ILLUMINATION DIFFUSER further teaches the use of particular colors for improving the illumination applicable to certain types of surfaces. Often, the choice of bright field, dark field, direct or diffuse light is not intuitive to user for many types of surfaces and/or the particular angles at which the reader is directed toward them. In other words, a surface may appear to be best read using dark field illumination, but in practice, bright field is preferred for picking out needed details, especially at a certain viewing angle. Likewise, with handheld readers, the viewing angle is never quite the same from surface to surface (part-to-part) and some viewing angles be better served by bright field while other may be better served by dark field. The above-referenced patent applications contemplate the application of a plurality of illumination types to achieve the best image for a particular surface and viewing angle.
It has been recognized that handheld readers pose a number of unique concerns. At least some of these concerns are shared in relation to fixed readers. For example, the material from which most light pipes are constructed is acrylic (commonly termed “plexiglass”). Acrylic exhibits a high refractive index (approximately 1.58), which is well suited for internal transmission of light down a light pipe. However, acrylic tends to shatter easily in response to impact. This may limit the life and endurance of a handheld reader (particularly a cordless/wireless model) that is expected to occasionally drop and strike a hard floor, perhaps against the light pipe. While the light pipe could be armored with cushioning and external housings, this undesirably increases production costs, weight, obtrusiveness and may optically obscure the pipe.
Moreover, the light pipes described in the above referenced patents may include a chamfered end to project dark field illumination via internal reflection. Refraction through the polished chamfered end also generates direct bright field illumination. The optical clarity of the light pipe and end tends to create a spotlight effect, in which each individual illumination source (red LEDs, for example) is clearly visible on certain surfaces (see FIG. 7 below). This controverts the typical goal of providing an even spread of illumination.
Also, where a conical diffuser is employed to provide an overall source of direct diffuse illumination, prior art devices are limited in their ability to spread light from a few individual illumination sources (LEDs, for example) throughout the diffuser surface, and then onto the subject as diffuse light. Thus, the diffuse light tends to exhibit a characteristic, localized light spot and dark spot effect. Adding further illumination sources to the diffuse section may be limited both by space and the relative cost of illumination sources, particularly where relatively costly blue-colored LEDs are employed.
Further, prior art readers often include visual indicators located at their back, top or another surface that denote the current status of the reader (for example, power on/off, good read, error, bad read, ready, not-ready, etc.). Various information can be presented to the user via different color lights (red/green, for example) and/or via blinking patterns. However, in a production environment, small, rear-mounted or top-mounted indicators may be overlooked or present a distraction while the user tries to focus on the surface being read. A technique for more-conveniently integrating indicators with the user's main point of interest is highly desirable.